The present invention relates to an electromagnetic pickup and more particularly to a pickup employed in a crank angle detecting system of an engine for a motor vehicle.
The crank angle detecting system is provided for detecting the crank angle dependent on the angular position of a crankshaft of the engine. A disk having a plurality of projections on the periphery thereof is secured to the crankshaft, and an electromagnetic pickup is provided for sensing the position of the projections. The detected crank angle is used for controlling ignition timing and fuel injection timing of the engine.
FIG. 19a shows an electromagnetic pickup employed in a system for ignition timing control. When a pulse generator disk 101 rotates, an electromagnetic pickup 102 detects the angular position of projections on the disk 101 and produces a signal which is applied to a control circuit 103. In the control circuit 103, an output signal of the electromagnetic pickup 102 is converted into a pulse signal by a waveform shaping circuit. The control circuit 103 calculates ignition timing and engine speed based on the crank signal in accordance with an input signal from a load sensor 104 such as an intake passage pressure sensor and produces a timing signal which is applied to an ignition device 105.
When the pulse generator disk 101 rotates, the projection passes the electromagnetic pickup 102 with a predetermined clearance L so that the magnetic flux between the disk 101 and the electromagnetic pickup 102 varies. Thus, the electromagnetic pickup 102 generates electromotive force having a voltage e in proportion to the variation of magnetic flux d.phi./dt. The output voltage e is compared with a reference voltage R in the waveform shaping circuit as shown in FIG. 19b and a crank signal P in the form of a pulse is produced as shown in FIG. 19c.
Since the disk 101 and the electromagnetic pickup 102 are provided in the engine, induced voltage in accordance with the variation of an external magnetic field caused by a current for a starter and a charging current from an alternator enters the output of the electromagnetic pickup 102 as noise. Accordingly, a pulse signal due to the noise is produced as shown in FIG. 19c.
Japanese Patent Application Laid-Open 57-186102 discloses a magnetic pickup which operates to eliminate noises, thereby producing only the necessary output signal.
However, the pickup eliminates the only noise which has a lower voltage than the output voltage of the magnetic pickup. Accordingly, if the external magnetic field is largely changed near the magnetic pickup, a noise having a large peak value enters the output of the magnetic pickup, which cannot be eliminated from the output. Consequently, proper ignition timing or correct engine speed is not calculated, which causes malfunction of the engine, and hence reduction of engine power and increase of fuel consumption.